1. Field of the Invention
The present invention relates to a holding sealing material, an exhaust gas purifying apparatus, and a method for manufacturing a holding sealing material.
2. Discussion of the Background
Particulate matter (hereinafter, also referred to as “PM”) contained in exhaust gases discharged from internal combustion engines such as diesel engines has raised serious problems as contaminants harmful to the human body. Also, harmful gas components such as CO, HC or NOx contained in exhaust gases have raised problems as contaminants harmful to the environment and the human body.
In light of those problems, as an exhaust gas purifying apparatus which is connected to the internal combustion engine to capture PM in exhaust gases or which converts toxic gas components such as CO, HC and NOx contained in exhaust gases, there are disclosed a variety of exhaust gas purifying apparatuses including an exhaust gas treating body including a porous ceramic such as cordierite or silicon carbide, a casing to accommodate the exhaust gas treating body inside thereof, and a holding sealing material disposed between the exhaust gas treating body and the casing.
Those holding sealing materials mainly include inorganic fibers. The exhaust gas treating body is held by the effect of elasticity of the inorganic fibers so that the exhaust gas treating body is prevented from coming off due to pressures applied by the exhaust gasses.
In these days, internal combustion engines are operated at an air fuel ratio close to the theoretical air fuel ratio for the purpose of improvement in fuel consumption, and thus temperatures of the exhaust gases are likely to be high. Particularly, when internal combustion engines are driven at a range of high rotational rates, the temperature of exhaust gases immediately after emitted from the combustion engine may rise to as high as about 1000° C. Therefore, the high temperature exhaust gases reaching to the exhaust gas purifying apparatus may increase the temperature of the exhaust gas purifying apparatus to a high temperature.
Therefore, the holding sealing material forming one component of the above-mentioned exhaust gas purifying apparatus is required not to be damaged by erosion loss and the like even under a high temperature condition at about 1000° C.
As a holding sealing material for use under a high temperature condition, there is proposed a holding sealing material using inorganic fibers (hereinafter also simply referred to as alumina fiber) mainly containing alumina.
Generally, among inorganic fibers, the alumina fiber has a relatively high heat resistance and high wind erosion resistance but has a low elasticity. Accordingly, the holding sealing material including only alumina fibers has low properties of holding the exhaust gas treating body (hereinafter simply also refers to as holding properties). Therefore, in order to provide the holding sealing material including only alumina fiber with high holding properties, a large amount of the alumina fiber needs to be used.
It is to be noted that the alumina fiber is relatively expensive among inorganic fibers. Therefore, in the case of the holding sealing material using a large amount of alumina fiber, the holding sealing material becomes expensive. As a result, the exhaust gas purifying apparatus using such a holding sealing material may become expensive.
In order to solve those problems, for example, there is disclosed a holding sealing material including inorganic fibers mainly containing heat-treated silica (hereinafter also simply referred to as silica fiber) (JP-T 2006-501402)
Further, there is proposed a holding sealing material including two inorganic fiber layers of an alumina fiber layer and an inorganic fiber layer mainly including alumina and silica (hereinafter, the inorganic fiber layer is also simply referred to as ceramic fiber) (JP-A 2004-204819).
The holding sealing material disclosed in JP-T 2006-501402 includes silica fibers whose elasticity has been improved by heating the silica fiber that is relatively inexpensive among inorganic fibers.
The holding sealing material disclosed in JP-A 2004-204819 includes two inorganic fiber layers of the alumina fiber layer and the ceramic fiber layer. The holding sealing material is said to have wind erosion resistance as the same with the holding sealing material including only alumina fibers.
The content of JP-T 2006-501402 and JP-A 2004-204819 are incorporated herein by reference in their entirety.